Is LED backlit (in LED monitors) flicker-free?


M

Man-wai Chang

I meant, is it continuous, or is it flashing like CCFL?

--
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J

John Doe

I mean... Why does Hong Kong and its people think that UseNet is
for selling their namebrand knockoff products?

How many aliases does the original poster have and use for
spamming UseNet?

Only The Shadow knows...
--
 
V

VanguardLH

Man-wai Chang said:
I meant, is it continuous, or is it flashing like CCFL?

Here's the question.
--
@[email protected] Might, Courage, Vision, SINCERITY.
/ v \ Simplicity is Beauty! May the Force and Farce be with you!
/( _ )\ (x86_64 Ubuntu 9.10) Linux 2.6.35.4
^ ^ 20:41:01 up 16 days 22:48 1 user load average: 0.02 0.05 0.01
ä¸å€Ÿè²¸! ä¸è©é¨™! ä¸æ´äº¤! ä¸æ‰“交! ä¸æ‰“劫! ä¸è‡ªæ®º! è«‹è€ƒæ…®ç¶œæ´ (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

Oh no, I forgot what was the question after having to wade through a
signature that comprises 78% of the lines in the post consisting of
random fluff quotes, proselytizing linux, and pointing at a particular
gov't agency in a worldwide publicized post.
 
P

Paul

Man-wai Chang said:
I meant, is it continuous, or is it flashing like CCFL?

I can't answer the question, but I can make a few suggestions.
My guess is, it is "flashing".

First of all, LEDs give a relatively consistent color output, if
operated at a constant current. If you attempt to control their
intensity, by changing the current flowing in them, the color
shifts. And good color control is necessary to get a nice white
balance from the backlight.

The next alternative, is called PWM or pulse width modulation.
A constant current is delivered to the LED, for a variable period
of time. The modulation will be at a frequency above the ability
of humans to perceive. For the LED and control circuit, it
probably isn't that critical what the frequency is - in other
words, it is easy for them to make it high enough so you
won't see a flicker.

You might ask, why would intensity adjustment be necessary at all ?
Two reasons.

1) Modern monitors have dynamic contrast specifications. When you
view a movie, the backlight is turned down on demand, frame by frame,
to give the best contrast ratio. This allows 3000:1 contrast from
LCD panels that have only 500:1 contrast from the panel itself.
It allows the quotation of an exaggerated contrast spec.

2) LEDs age with time. A LED will lose 50% of intensity within the
first two years of service. To compensate for the loss of
intensity, a photodetector in the LCD monitor can detect the shift, and
increase the pulse width period to compensate. The compensation
can be applied during the entire life of the monitor. The backlight
level may also be adjusted, according to ambient lighting
conditions in the room.

The old LCD monitors, with their CCFL illumination sources
do much the same thing. CCFLs are modulated at two frequencies.

1) The inverter runs at 25KHz or so. It converts 12VDC to 700-1000 VAC
to start the CCFL running. The CCFL tube runs at a high voltage.
The reason the CCFL inverter runs at 25KHz, is so the noise from
the inverter operation, will be above the range of human hearing.

2) For intensity control, the inverter is run in bursts, at about 200Hz.
The exact frequency is selected, so you won't perceive a flicker.
(It's that old PWM thing again :) ) This provides a wider range
of light intensity settings, than simply varying the 12VDC fed
into the inverter. And it also allows the CCFL to run with a
consistent coloring (at least, until the tube wears out and
the light turns brownish).

So in some ways, they're equivalent technologies, with similar capabilities.
The ability to provide dynamic contrast, the ability to compensate for
loss of light output with age. And relative freedom to select operating
frequencies which won't be perceived by humans (either visually or
aurally).

The LED provides an advantage on color gamut. It's unclear, whether
in all cases, the LED monitor is really saving a lot of power. I
haven't seen measurement numbers to back up such a statement. And
the LED might last longer, because it isn't dependent on a dirt-cheap
inverter for its operation. That potentially makes it a more reliable
technology. The reliability, really depends on the manufacturing,
and how many corners they cut while making the product.

(Extended Gamut)
http://www.xbitlabs.com/images/monitors/samsung-sm-xl24-xl30/p2s.png

Paul
 
J

John Doe

Charles_Schilli said:
Having a spam address in more places improves the search rating
for the spam.

That depends on the context. If that were entirely true, they
would be rich.
Please trim it out when/if responding.

I have been messing with spam for a long time here on UseNet.
("Why Hong Kong"? Internet connection and need for cash. What do
you think?)

No telling what that stuff is supposed to mean.
--
 
M

Man-wai Chang

LEDs age with time. A LED will lose 50% of intensity
within the first two years of service. To compensate
for the loss of intensity, a photodetector in the LCD
monitor can detect the shift, and

Does this mean all LED monitors, though cheaper to make, require
replacement every few years?

--
@[email protected] Might, Courage, Vision, SINCERITY.
/ v \ Simplicity is Beauty! May the Force and Farce be with you!
/( _ )\ (x86_64 Ubuntu 9.10) Linux 2.6.35.4
^ ^ 19:09:01 up 19 days 21:16 1 user load average: 0.06 0.05 0.00
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P

Paul

Man-wai Chang said:
Does this mean all LED monitors, though cheaper to make, require
replacement every few years?

The LED continues to operate, so it is not a problem. There
is a graph here, to show the relative loss in intensity of
various light sources, for comparison. Every generation of
LEDs, has different properties in this regard.

http://www1.eere.energy.gov/buildings/ssl/depreciation.html

I just looked at a recently introduced high power LED, and
it lists "70% lumen maintenance (L70) at 50,000 hours of operation",
and 50000 hours is a decent lifetime.

Paul
 

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